Color digital halftoning using black and secondary color replacement and color vector dithering

Abstract

Provided is a system and method for selectively enabling a printer to deposit on a sheet cyan, magenta, yellow and black (C,M,Y and K) primary or black color dots at each of the plurality of pixel locations to produce a color image. The system comprises a memory for storing C, M, Y and K values for each pixel in the color image and a processor for controlling selective deposition of the C, M, Y and K colors. The system includes a memory for holding a color vector for each pixel, respectively in the color image; a memory for storing parameters of a dither cell; and a processor coupled to the memory for comparing the dither cell with each color vector, to select a color component to print at the corresponding pixel location based on the comparison. The system provides a halftoning technique that produces print quality near error diffusion quality with the speed of a dither by dithering on a color vector. The replacement of black and secondary colors by the system produces better print quality than other dither techniques.

What is claimed is: 
     
       1. A system for selectively enabling a printer to deposit on a sheet primary or black color dots at each of a plurality of pixel locations, to produce a color image, said system comprising; first memory means for holding a color vector for each pixel, respectively, in the color image;   second memory means for storing parameters of a dither cell;   processor means coupled to said memory means for comparing said dither cell with each color vector, to select a color component to print at the corresponding pixel location, based on said comparison.   
     
     
       2. The system of claim 1, wherein: the first memory means and the processor means together comprise means for comparing the dither cell with nested subvectors, respectively made up of nested subcombinations of the color components in each color vector.   
     
     
       3. The system of claim 2, wherein: the comparing means identify (1) the smallest subvector which is larger than a corresponding dither-cell value, and (2) a particular color component which is in that subvector but not in the next smaller subvector; and   the processor means select that component to print.   
     
     
       4. The system of claim 3, wherein: the subvector subcombinations are nested in a specified nesting sequence of the color components.   
     
     
       5. The system of claim 4, wherein: the specified nesting sequence comprises black in the smallest nonzero subvector, and white in the largest.   
     
     
       6. The system of claim 5, wherein: the specified nesting sequence comprises a secondary color in the next-to-said-smallest subvector.   
     
     
       7. The system of claim 6, wherein: the specified nesting sequence comprises primary colors in the subvectors immediately larger than the next-to-said-smallest.   
     
     
       8. The system of claim 7, wherein: the specified nesting sequence comprises the primary colors in order of increasing lightness.   
     
     
       9. The system of claim 7, wherein: the primary colors are magenta, cyan and yellow; and   the specified sequence comprises the primary colors in that order.   
     
     
       10. The system of claim 9, wherein: the secondary colors are red, green and blue; and   the printer prints the secondary colors as combinations of the primary colors.   
     
     
       11. The system of claim 1, wherein: the second memory means comprise means defining a mask value for each pixel position in the dither cell;   the processor means comprise means for establishing which pixel position in the dither cell corresponds, for purposes of comparison, with each color vector;   the first memory means and the processor means comprise means for comparing the mask value in each pixel position in the dither cell with nested subvectors, respectively made up of components of the corresponding color vector.   
     
     
       12. The system of claim 11, wherein the establishing means comprise means for: first, in effect positioning the dither cell in a corner of the plurality of pixel locations;   then in effect shifting the dither cell along a first direction until there are no more pixel locations in that direction;   then in effect shifting the dither cell along a second direction, substantially orthogonal to the first direction, and shifting the dither cell along the first direction until again there are no more pixel locations in that first direction; and   then repeating the steps recited in the preceding paragraph until there are no more pixel locations along the second direction.   
     
     
       13. The system of claim 1, wherein: the printer deposits drops of ink that are black and chromatic primaries;   the chromatic primaries are cyan, magenta and yellow; and   the printer prints chromatic secondaries as combinations of the primaries.   
     
     
       14. The system of claim 1, further comprising: means for receiving information about a desired color image and in response thereto generating said color vector;   wherein the receiving-and-generating means comprise means for:   expressing the desired color image information in the form of an input color vector for each pixel position, if the information is not already in that form; and   replacing a fraction of the black in each input color vector with chromatic colors, to form the first-mentioned said color vector.   
     
     
       15. The system of claim 14, wherein the receiving-and-generating means further comprise: means for removing a quantity of white in conjunction with said black replacement; and   wherein the processor means comprise means for setting said fraction of the black component to a value between zero and one inclusive, depending upon the quantity of white that is available for removal.   
     
     
       16. The system of claim 15, wherein: the receiving-and-generating means further comprise means for, in some color vectors, replacing at least part of a secondary color, if present, with primary colors.   
     
     
       17. The system of claim 16, wherein: the receiving-and-generating means further comprise means for removing a further quantity of white to compensate for said secondary-color replacement; and   said some color vectors are those in which said further quantity of white remains available after the black replacement.   
     
     
       18. The system of claim 1, further comprising: means for translating said selected color at each pixel location into a form usable by said printer.   
     
     
       19. The system of claim 18, wherein: said form usable by said printer comprises red, green and blue color coordinates.   
     
     
       20. A method of operating a color printer to print a desired image as an array of dots of colored ink at a large number of pixel positions, comprising the steps of: receiving information about the desired color image for each pixel position;   expressing the desired color image information in the form of a color vector for each pixel position, if the information is not already in that form;   replacing at least part of the black in each color vector with chromatic colors;   removing a quantity of white to compensate for said black replacement;   replacing at least part of a secondary color, if present, in each color vector with primary colors, provided that at least a certain further quantity of white remains available after said black replacement;   removing said further quantity of white to compensate for said secondary-color replacement, if secondary-color replacement has been performed;   wherein said expressing, replacing and removing steps produce a resulting color vector;   comparing nested subvectors respectively made up of color components in the resulting color vector, at each pixel position, with a corresponding dither-cell value for that pixel position;   said nested subvectors being assembled in a sequence comprising black, then a secondary color, then the primary colors in order of increasing lightness, and then white;   selecting for printing, at the corresponding pixel location, the first component of the color vector which produces a cumulative aggregate greater than said corresponding dither-cell value;   translating said selected color vector at each pixel location into output color coordinates usable to control the printer; and   applying the output color coordinates to control the printer.